1. Field of the Invention
The present invention relates to the intravascular-ultrasound (IVUS) arts, and more particularly to a system and method of (i) using ultrasound data backscattered from vascular tissue to estimate the transfer function of a catheter and/or (ii) substantially synchronizing the acquisition of blood-vessel data to an identifiable portion of heartbeat data. Reference is made to related subject matter in U.S. patent application Ser. No. 10/758,477, filed Jan. 14, 2004, and U.S. patent application Ser. No. 10/647,977, filed Aug. 25, 2003, which claims the benefit pursuant to 35 U.S.C. §119(e) of U.S. Provisional Patent Application Nos. 60/406,183, 60/406,254, 60/406,148, 60/406,184, 60/406,185 and 60/406,234, all of which were filed Aug. 26, 2002, which applications are specifically incorporated herein, in their entirety, by reference.
2. Description of Related Art
Ultrasound imaging of the coronary vessels of a patient can provide physicians with valuable information. For example, such an image may show the extent of a stenosis in a patient, reveal progression of disease, determine the vulnerability of the atherosclerotic plaque for causing myocardial infarction, help determine whether procedures such as angioplasty or atherectomy are indicated, or whether more invasive procedures are warranted.
In a typical ultrasound imaging system, a catheter (including an ultrasonic transducer attached thereto) is carefully maneuvered through a patient's blood vessel to a point of interest. Acoustic signals are then transmitted and echoes (or backscatter) of the acoustic signals are received. The backscattered ultrasound data (“backscattered data”) can be used to identify the type or density of the tissue being scanned. When the echoes (or multiple sets thereof) are received, acoustic lines are processed, building up a sector-shaped image of the blood vessel. After the backscattered data is collected, an image of the blood vessel (i.e., an intravascular-ultrasound (IVUS) image) is reconstructed using well-known techniques. This image is then visually analyzed by a cardiologist to assess the vessel components and plaque content.
A first drawback of such a system, however, is that the ultrasound data backscattered from the vascular tissue may not accurately represent the tissue. This is because the backscattered data may further include a noise component and a catheter component. For example, with respect to the latter, manufacturing tolerances can cause different catheters (or devices connected thereto—e.g., IVUS console, transducer, etc.) to operate differently (e.g., at slightly different frequencies, etc.), thus producing different results. This influence on the system is referred to herein as the “transfer function.”
Traditionally, the transfer function has been determined (i) with the catheter outside the patient and (ii) through the use of a perfect reflector (e.g., plexiglass, etc.). Specifically, the catheter would be positioned near the reflector and used to transmit ultrasound data toward the reflector and to receive ultrasound data backscattered from the reflector. Because the reflector backscatters all (or substantially all) of the data transmitted, the catheter's transfer function can then be computed. This is because the backscattered data (B) is equal to the transmitted data (T) as modified by the transfer function (H) (i.e., B=TH). Drawbacks of such a system, however, are that the transfer function cannot be computed in real-time (e.g., while data backscattered from vascular tissue is being acquired, etc.) and requires the use of additional components (e.g., a perfect reflector, etc.). Thus, it would be advantageous to have a system and method for determining the transfer function of a catheter that overcomes at least one of these drawbacks.
A second drawback of such a system, is that large amounts of backscattered data are often acquired but not used, thus resulting in an unnecessarily large memory device. For example, a patient's blood vessels are continuously expanding and relaxing in response to blood being pumped there through. Thus, by continuously gathering backscattered data, a blood vessel, as it expands and relaxes, can be imaged. Often, however, it is necessary to image the blood vessel in a particular position (e.g., as if it were standing still, or not expanding and relaxing).
The traditional method of doing this (at least with respect to an IVUS device) is to gather both blood-vessel and heartbeat data (e.g., EKG data), use the blood-vessel data to generate real-time images (i.e., video of the expanding and contracting blood vessel), record these images onto a VHS tape, and use a computer and the heartbeat data to extract relevant frames from the VHS tape. The heartbeat data is used by the computer because the rhythm of the heart is related to the expansion and contraction of the blood vessels. Thus, by extracting the frames recorded during an identifiable period in the heart's cycle, the blood vessel can be monitored as if it were standing still, or not expanding and contracting.
Not only does this result in an image having a lower resolution (due to the VHS recording), but it requires a large memory device for storing unnecessary backscaftered data and/or frames related thereto. Thus, a need exists for a system and method of acquiring backscattered data from a blood vessel in a particular position that overcomes at least one of these drawbacks.